Outfitting a buddy of mine and we're looking at sleeping bags. He's 5'6". I'm wondering if he will experience any significant heat loss using a standard bag rather than a short bag sized for someone to 5'6" or so. Not all bags come in short sizes and I'm wondering if we need to focus on only those bags that do or if he'd be alright with the standard size. Most of my experience is with quilts.

I think that's a valid concern because he'll be heating more internal volume. That said, he can customize the bag length simply by pushing the additional length back up inside the bag and either tying it off or sewing it off and also will result in more insulation around the feet. I'm 5'5" and switched to a long bag this year because last year I got tired of my feet being jammed into the end of the foot box. I think a lot of people don't leave room for the fact that your feet tend to sort of flop down when you sleep. I made that mistake, went longer this year, and will just stuff the extra that I don't want up inside the foot box for more insulation. Once I figure out how I want it, I'll sew it off at that length.

What Dena said. You have to measure your size while lying down. Compressed insulation near the feet is a bad idea, because it tends to produce cold feet. Also keep in mind that you might want to take stuff inside the bag (clothes to dry, down booties, etc.) and a little extra room helps for that.

But sleeping position is also quite relevant. Side sleepers can probably get away with a bag that is a little short but a little wider.

One thing last year's nasty case of plantar fasciitis taught me is that when we sleep, our feet relax into the pointed toe position (why night splints are recommended for PF). It really depends on how you sleep. If you sleep on your back stretched out, you will take up a couple inches more than your height. In other words, anyone over 5'4" will not have enough foot room in a 5'6" bag. For those who always sleep curled on on the side, it might not be a big deal. People change, though. I've always been a side sleeper, but the last few years I have shifted to occasional back sleeping. I'm 5' 3 1/2" and my toes just touch the bottom of a short Western Mountaineering bag if I stretch out full length. If I were any taller, I'd definitely want a medium bag.

And always try a bag on while you can still return it; girth is an important size aspect of a sleeping bag, as is an individual's tolerance for how snug a bag can be tolerated.

A regular size bag is nice if you ever need to store water bottles or fuel canisters in them while you are sleeping. Also, if you are like most of us and sell your gear often, the regular length bag will be easier to sell.